To describe the range of clinical manifestations of Eastern Equine Encephalitis (EEE) cases in southern New England.

EEE virus (EEEV), a mosquito-borne alphavirus, causes the most lethal arboviral encephalitis in North America, with a reported mortality of 40%. EEEV is endemic to the eastern United States but human cases are rare as only 5% of EEV infections result in EEE. No cases were reported in southern New England between 2014 and 2018. Currently, there is no vaccine or effective treatment available for infection in humans.

We describe the clinical presentation, management and outcome of four cases of EEE treated in our tertiary care academic medical center during August and September of 2019.

We report a surge in cases of confirmed EEE, consisting of one six-year old girl and three men in their 50s. Mortality was 50%. Cerebrospinal fluid (CSF) was characterized by mild neutrophilic pleocytosis and elevated protein. In addition to the typical involvement of basal ganglia and the external capsule, the patients who died also had involvement of the pons and bilateral thalami. They rapidly progressed to poor neurological examination and coma. Electroencephalogram (EEG) in these patients showed severely suppressed frequencies and subsequent development of refractory status epilepticus. The adult patients were treated with intravenous immunoglobulin (IVIg), plasmapheresis, or both. The patients that survived recovered without neurological sequelae.

Although rare, EEE should be considered early in the differential diagnosis of patients presenting with deep gray matter involvement, pleocytosis (lymphocytic or neutrophilic), and elevated CSF protein in summer months in endemic areas. Patient outcomes vary widely based on disease progression. In our cases, major predictors of poor prognosis were involvement of bilateral thalami and brainstem, severely suppressed frequencies on EEG, and subsequent refractory status epilepticus.